1. Field of the Invention
The present invention relates generally to processes for semiconductor manufacturing and, more particularly to optical lithography and the determination of focal plane deviation (FPD) associated with photolithographic projection systems.
2. Outline of the General Theory
The semiconductor industry's requirement to produce smaller critical features over time has forced semiconductor manufacturers and lithography tool vendors to produce higher numerical aperture (NA) lithography systems (Steppers or Scanners) using smaller wavelengths (for example, 193 nm DUV lithography). The ability to produce (manufacture) sub-wavelength features can often be determined by considering the rather simple (3-beam) Rayleigh scaling Resolution (R) and Depth-of-Focus (DoF) equations; ˜λ/2NA and ˜λ/2NA2 respectively. These coupled equations stress the inverse relationship between resolution and DoF based on the exposure wavelength (λ) and numerical aperture (NA)—for features printed near the limit of the optical system. High NA lithography has led to improved resolution and a reduction in the overall focus budget—making lithography processes difficult to control. See, for example, “Distinguishing Dose from Defocus for In-line Lithography Control”, C. Ausschnitt, SPIE Vol. 3677, 140:147, 1999. Poor lithographic process control (focus and exposure) leads to smaller product yields, increased manufacturing costs, and poor time to market. While semiconductor lithographers have discovered creative reticle enhancement techniques (RETs) and other optical techniques to increase the useable DoF—the problem remains. See, for example, “The Attenuated Phase Shift Mask”, B. Lin, and “Method and Apparatus for Enhancing the Focus Latitude in Lithography”, Yan, U.S. Pat. No. 5,303,002 issued Apr. 12, 1994. Therefore, it is important to monitor focus during photolithographic processing and develop new methods for focus control. Typically focus error across a stepper field can be attributed to the following three factors: (1) wafer and reticle non-flatness, (2) wafer/reticle stage error, and (3) lens field curvature. For a photolithographic stepper, field curvature varies across the image field in x and y. There are many methods for determining/monitoring focal plane deviation (FPD) and best focus by field position for photolithographic exposure tools. See, for example, “Distinguishing Dose from Defocus for In-line Lithography Control”, supra; “A Simple and Calibratable Method for the Determination of Optimal Focus”, J. Gemmink, SPIE Vol. 1088, 220:230, 1989; “Astigmatism and Field Curvature from Pin-Bars”, J. Kirk, SPIE, Vol. 1463, 282:291, 1991; “Photo-lithographic Lens Characterization of Critical Dimension Variation Using Empirical Focal Plane Modeling”, M. Dusa, et al., SPIE, Vol. 3051, 1:10, 1997; “Latent Image Metrology for Production Wafer Steppers”, P. Dirksen et al., SPIE, Vol. 2440, p. 701, 1995; “Controlling Focal Plane Tilt”, S. Hsu et al., Semiconductor International—Online, 1998. None of these, however, accounts for wafer non-flatness independent of the lithographic process in an absolute sense. See, for example, “Competitive Assessment of 200 mm Epitaxial Silicon Wafer Flatness”, H. Huff et al., SPIE, Vol. 3332, 625:630.
The ability to precisely control the photolithographic stepper tool depends on the ability to determine the magnitude and direction of the individual focusing error components (see items 1-3 mentioned above). While focusing error causes reduction in image fidelity, the coupling of focus error and other lens aberrations (distortions) degrades overlay or positional alignment as well. See, for example, “Impact of Lens Aberrations on Optical Lithography”, T. Brunner.
Over the past 30 years the semiconductor industry has continued to produce faster (via smaller critical features) and more complex (greater functionality, dense patterning) circuits—year after year. See, for example, “Optical Lithography—Thirty Years and Three Orders of Magnitude”, J. Bruning, SPIE, Vol. 3051, 1997. The push to smaller feature sizes is gated by many physical limitations. As the critical dimensions of semiconductor devices approach 50 nm, the usable DoF will approach 100 nm. See, for example, “2001 ITRS Roadmap”, SEMATACH, 1:21. Continued advances in lithography equipment (higher NA systems, smaller wavelength exposure sources), RET's, resist processing, and automated process (focus and exposure) control techniques will likely become more difficult and remain critical. See, for example, “2001 ITRS Roadmap”, supra; “The Waferstepper Challenge: Innovation and Reliability Despite Complexity”, Gerrit Muller, Embedded Systems Institute Netherlands, 1:10, 2003.
FPD: There are a number of methods that, with greater or lesser accuracy, measure defocus or focal plane deviation (FPD) over an exposure field. In general terms, each of these techniques estimate the focal error across the field using a variety of special reticle patterns (focusing fiducials, FF), interferometric devices, mirrors, sensors, and statistical models. In addition, each of these methods utilizes the stepper wafer stage leveling and positioning system and/or optical alignment system to aide in the determination of FPD. FPD is a rather general term describing the complete focus error associated with the photolithographic stepper—deviations from the focal plane in reference to the wafer surface. Among other things, FPD can be caused by lens tilt, stage/reticle tilt, reticle bow and lens field curvature. FIG. 1 shows a generic photolithographic leveling system. FIG. 2 illustrates some common reticle patterns (the IBM Phase Shift Focus Monitor (PSFM), and the ASML FOCAL alignment mark) that are used to determine FPD for both steppers and scanners. Typically, FPD calibration/monitoring is performed daily or at least weekly to ensure that the stepper or scanner is operating within design limits (verifying the focus system works, the stage is level, etc.). While both techniques are Widely accepted, both techniques require complex calibrations to be performed at each field point. See, for example, “Detailed Study of Phase-Shift Focus Monitor”, G. Pugh et al., SPIE, Vol. 2440, 690:700, 1995; “Latent Image Metrology for Production Wafer Steppers”, supra.
Some FPD prior art methods are summarized below in Table 1:
MeasurementMethodTypeCommentISI (See, for example,AbsoluteExtremely accurate.“Apparatus Method ofMeasurement and Method ofData Analysis for Correction ofOptical System”, Smith et al.,U.S. Pat. No. 5,978,085,Nov. 2, 1999)FOCAL (See, for example,RelativePublished version“Latent Image Metrology forclaims high absoluteProduction Wafer Steppers”accuracy, resolutionsupra)averaging in practice.IBM focus monitor U.S. Pat.AbsoluteRequires ‘calibration’.No. 5,300,786It is very processindependent.Schnitzl (See, for example,Relative withComplex calibration,“Distinguishing Dose fromone exposurevarying targetDefocus for In-Line Lithographysensitivity.Control”, supra)
ISI (Litel Instruments): A method for determining the aberrations of an optical system is described in U.S. Pat. No. 5,978,085 to Smith, in which a special reticle is used to determine the Zernike coefficients for photolithographic steppers and scanners. Knowing the wavefront aberration (Zernike coefficients and the associated polynomial) associated with the exit pupil of the projection system includes information about the lens field curvature or focus (Zernike coefficient a4, for example). Smith in the '085 patent uses a special reticle and a self-referencing technique to rapidly identify FPD to a high degree of accuracy (determines focusing errors to ˜5 nm) in the presence of scanner noise. This method automatically determines lens field curvature information for both static and dynamic exposure tools (steppers and scanners).
PSFM: A method (called Phase Shift Focus Monitor) described by T. Brunner et al. in U.S. Pat. No. 5,300,786 entitled “Optical Focus Phase Shift Test Pattern, Monitoring System and Process”, can be used to determine and monitor the focal plane deviation (FPD) associated with the lithographic process. More information can be found in, for example, “Detailed study of a phase-shift focus monitor”, supra. In general, an alternating PSM with phase close to 90° possesses unusual optical properties that can be exploited to measure focus errors. See, for example, “Quantitative Stepper Metrology Using the Focus Monitor Test Mask”,T. Brunner et al., SPIE, Vol. 2197, 541:549; “Using the Focus Monitor Test Mask to Characterize Lithographic Performance”, R. Mih et al., SPIE, Vol. 2440, 657-666, 1995. It is possible to design a “box-in-box” overlay target using a phase shift mask pattern (referred to here as a focusing fiducial; see FIGS. 2 and 3), in which the measured overlay error is proportional to the focus error. Focal plane non-flatness is then determined by measuring the focusing fiducials across the lens field. Astigmatism information appears as differences between the delta-X overlay error and the delta-Y overlay error measurement. This technology has also been used for assessing variations in focus across the wafer due to lens heating, misfocusing near the edge of the wafer, and chuck/stage non-flatness. One major drawback with the PSFM method is that a fairly elaborate calibration procedure (focus offset vs. overlay shift for each field point) is required before it can be used—the PSFM technique is rather sensitive to the source-sigma (Na-source/Na-objective) that varies process to process. Additional PSM techniques, such as those found in, for example, “Focus Monitor for Alternating Phase Shifted Masks”, Liebmann et al., U.S. Pat. No. 5,963,738 issued Aug. 10, 1999, are used in a similar way. While the PSFM method provides an FPD map across a scanner or stepper field, it does not provide a method for determining the dynamic lens field curvature independent of wafer height variation in the presence of stage synchronization error. See, for example, “Comprehensive Focus-Overlay-CD Correlation to Identify Photolithographic Performance”, Dusa et al., SPIE, Vol. 2726-29, 1996.
FOCAL: A method called “FOCAL”, for Focus determination using stepper alignment system, described by P. Dirksen, et. al., SPIE Vol. 2440, 1995, p. 701, specifies a focusing fiducial that can be used to find FPD and astigmatism across the exposure field (lens). FOCAL alignment marks (focusing fiducials) consist of modified wafer alignment marks (FIG. 1 of “Latent Image Metrology for Production Wafer Steppers”, supra) that are measured using the stepper wafer alignment subsystem. Defocus of the tool results in an apparent shift of the center of the alignment mark relative to that of the “best focus” position. The FOCAL technique makes use of the exposure tool's alignment mechanism and therefore requires that the stepper or scanner be off-line for the length of the measurement sequence. FOCAL marks are sensitive to exposure and sigma like the PSFM method; however, since fiducial response is a function of pitch, the target features are less dependent upon reticle error. Furthermore, the FOCAL data (focus vs. overlay error) must be calibrated for every point in the exposure field similar to phase-shift monitors (typically at 121 points across an exposure field—see FIG. 11). Now, it is possible to use FOCAL to separate out lens tilt and astigmatism from dynamic FPD maps and provide a dynamic focal plane map, but wafer height variation and stage synchronization errors would still be included in the result See, for example, “193 nm Step and Scan Lithography”, Davis et al., SEMI Technology Symposium 98, and “Comprehensive Focus-Overlay-CD Correlation to Identify Photolithographic Performance”, supra.
Schnitzl Targets: A method described by Ausschnitt makes use of line-end shortening effects to decouple focus drift from exposure drift on semiconductor product wafers. See, for example, “Distinguishing Dose from Defocus for In-Line Lithography Control”, supra. FIG. 3 shows a typical pair of Schnitzl targets (focusing fiducials). It is widely known that resist line-ends (FIG. 3) are very sensitive (exhibit greater line-end shortening) to both focus and exposure drifts; the effect is further enhanced as the lithographic process is pushed near performance limit of the scanner tool (˜λ/2NA). Using the Schnitzl targets and a fairly elaborate method of calibration (CD-SEM measurements and a coupled system of equations), Ausschnitt offers a method that can determine the magnitude of focus drift on product wafers using one or more exposures in the presence of exposure drift. Since changes in focus and exposure can produce similar changes in the critical dimension (CD), the Schnitzl method is useful for day-to-day process monitoring because it eliminates the need to constantly perform focus and exposure experiments (such as FEM, a Focus Exposure Matrix) in-between production runs. In addition, the method uses fast and accurate optical overlay tools to measure the Schnitzl patterns (in several forms, CD targets or Overlay targets, FIGS. 2-3) after wafer processing. This saves monitoring costs because optical overlay tools are less expensive to operate as compared with a CD-SEM. While decoupling focus drift from exposure drift is useful for process monitoring, the method in its present form requires two exposures at different focus settings to determine the absolute focal drift (direction). Performing extra exposures during production runs is very costly. In addition, since the initial Schnitzl target calibration procedure depends on a number of lithographic tool settings (line size, pitch, sigma, NA) re-calibration is required for each lithographic process change—including changes in metrology tools. The Schnitzl focusing fiducials are often used to map out FPD across a stepper or scanner field, but methods similar to those in “Comprehensive Focus-Overlay-CD Correlation to Identify Photolithographic Performance”, supra, would need to be implemented to obtain a dynamic focus map—but again, wafer height variation and scanning dynamics are not considered.
Summarizing:
Several methods for determining FPD have been described. Common to all of these methods is that a feature (a focusing fiducial or FF) is printed on a wafer and the focusing fiducial is subsequently measured. The data from the focusing fiducial is processed and an FPD value, δZ, determined. Further, and common to all these methods, the contributions of wafer height, lens aberrations (in the form of lens field curvature), and stage Z-error are not resolved into their distinct components.